This invention relates generally to reactor pressure vessels and more specifically to reactor pressure vessels including shell courses containing nozzles that are fabricated from a single one-piece forging.
Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about 60 feet long, about twenty feet in diameter, and about seven inches thick. Because of its length, the shell is formed from a plurality of rings or shell courses welded together. The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell.
A plurality of nozzle openings are formed in the cylindrical shell for attachment of the pressure vessel to external pipes. Structural design standards dictate that when an opening is made in a pressure vessel, reinforcing material must be added around the opening. In addition, any weld joint used to attach nozzle reinforcing materials and connecting pipes need to be 100% volumetric inspectable.
Known pressure vessel shell courses are made in sections from either a one-piece forging or formed and welded plate. The sections are either welded or bolted together to form the complete vessel. When the shell courses are fabricated from formed and welded plate, longitudinal weld joints are formed that have increased stress levels compared to circumferential weld joints. The nozzles are made from forgings, which are welded into bores in the cylindrical sections, or shell courses, of the pressure vessel. The nozzle forging contains the reinforcing material necessary to ensure the integrity of the nozzle, and weld preps for the nozzle-shell and nozzle-pipe weld joints.
Periodically, known pressure boundary welds in nuclear pressure vessels are volumetrically inspected using ultrasonic testing to determine weld integrity. Accordingly, inspection equipment is required and the plant is configured to provide access for inspection.
Known pressure vessels and processes for making pressure vessels have several disadvantages. First, the nozzle forging to shell welds must be periodically inspected. This inspection process is time consuming and utilizes large amounts of labor and financial resources. In addition, the reliability of the reactor pressure vessel depends on the integrity of the individual nozzle forging to shell welds.
It is therefore desirable to provide a pressure vessel that includes fewer welded joints than current pressure vessels, meets all stress requirements and which eliminates vertical weld joints.
In an exemplary embodiment, a pressure vessel for a nuclear reactor includes at least one unitary forged nozzle shell course. Unitary forged nozzle shell courses simplify the reactor pressure vessel fabrication process and reduces weld joints in the pressure vessel. The pressure vessel has a substantially cylindrical shape and includes a plurality of ring shaped shell courses welded together. At least one shell course is a forged shell course that includes at least one reinforcement portion having an enlarged thickness that extends radially outward, and a nozzle machined into the reinforcing portion of the forged shell course wall. A bore extends from an inside surface of the forged shell course to the outer end of the nozzle. The reinforcing portion and the nozzle are machined from one ring forging of sufficient thickness to form a unitary shell course that includes at least one nozzle reinforcement projecting from the outer wall. The nozzle has a radius Rn, and the nozzle bore has a radius Rnb. The reinforcement portion includes a longitudinal dimension and a circumferential dimension. The longitudinal dimension is about two times the nozzle radius Rn, and the circumferential dimension is about 1.5 times the nozzle radius Rn.
The forged nozzle shell course is fabricated by providing a ring forging having a desired inside diameter and a thickness that is equal to a desired shell wall thickness plus at least the thickness of the reinforcing portion. The nozzle is formed in the forged nozzle shell course by machining the ring forging to form a reinforcing portion projecting radially outward from the shell wall and a nozzle machined into the reinforcing portion. The nozzle bore, having radius Rnb, is machined to be coaxial with the nozzle and to extend from the outer end of the nozzle through the shell wall to the inner surface of the shell wall.
The above described forged nozzle shell course eliminates pressure boundary weld joints between separate nozzle forgings and the shell course and therefore, provides a reactor pressure vessel with a reduced number of weld joints that need to be inspected during service. A reduced number of welded joints improves structural integrity of the RPV. Also, the above described forged nozzle shell course simplifies the reactor pressure vessel fabrication process and eliminates the need for inspections of nozzle to shell welds.